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THINK ABOUT IT

A silvery or reddish mineral called hematite, Fe₂O₃, is the main source of iron metal on Earth. When this iron (III) oxide is decomposed to iron and oxygen, the oxygen atoms lose electrons. This means that the iron atoms gain electrons.

What happens to electrons during oxidation?

To answer this question, you will explore

EXPLORING THE TOPIC

Oxidation-Reduction

When iron oxides are decomposed to iron metal, the iron cations gain electrons and the oxygen anions lose electrons.

2Fe₂O₃(s) → 4Fe(s) + 3O₂(g)

The oxygen anions in Fe₂O₃(s) have a charge of –2. In the decomposition of Fe₂O₃(s), each O^2– anion loses two electrons to form neutral oxygen atoms. The oxygen atoms bond in pairs to form elemental oxygen, O₂ (to satisfy the octet rule). In this process, the oxygen anions, O^2–, are oxidized to elemental oxygen, O₂.

The iron cations in Fe₂O₃(s) have a charge of +3. The iron atoms in Fe(s) have a charge of 0. In the decomposition of Fe₂O₃(s), each Fe³⁺ cation gains three electrons to form neutral iron atoms in the elemental metal. Just as there is a term for losing electrons (oxidation), there is a term for gaining electrons: reduction. In this process, the iron atoms, Fe³⁺, are reduced to elemental iron, Fe.

Whenever metals are extracted from their compounds, they are reduced. While the term reduction refers to the process of gaining electrons, its origins have to do with the concept of reducing metal ores to their simplest elements. Whenever you see a metal element as the product of a reaction, you can be fairly certain that it has been reduced.

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Every time a substance loses electrons, another substance gains electrons and vice versa. These two processes always take place together: oxidation and reduction. Oxidation-reduction reactions are often referred to as redox reactions.

REACTION OF ZINC AND COPPER SULFATE

A strip of zinc metal is placed in a light blue solution of copper (II) sulfate, CuSO₄, as shown in the illustration. Soon, a reddish coating begins to form on the zinc strip. The blue color of the solution begins to fade. What is going on?

Start with the balanced chemical equation for this reaction:

Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)

You know from Unit 1: Alchemy that when salts dissolve in water, the compound dissociates into ions. Often, chemical equations are written in ionic form to specify the charges of the ions in solution as shown here.

Notice that the sulfate ion is a spectator ion. The net ionic equation is therefore

Zn(s) + Cu²⁺ → Zn²⁺ + Cu(s)

The next illustration shows this reaction from a particle view. Take a moment to follow the transfer of the electrons.

Redox Reactions

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Many chemical reactions are oxidation-reduction reactions. In fact, any reaction in which electrons are transferred from one reactant to another is a redox reaction. This includes most reactions that have an element as a reactant or product, which are often single exchange reactions.

Typically, double exchange reactions are not redox reactions. For example, in acid-base reactions, H⁺ trades places with another cation. In precipitation reactions, an aqueous ion from one compound combines with an aqueous ion from another. So, double exchange reactions often involve transfer of ions, while single exchange reactions involve transfer of electrons.

SINGLE EXCHANGE: REDOX REACTION

Mg(s) + 2HCI(aq) → H₂(g) + MgCI₂(aq)

DOUBLE EXCHANGE: ACID-BASE REACTION

NaOH(aq) + HCI(aq) → H₂O(I) + NaCI(aq)

DOUBLE EXCHANGE: PRECIPITATION REACTION

AgNO₃(aq) + KCI(aq) → AgCI(s) + KNO₃(aq)

Oxidation-reduction reactions are not limited to metals and ionic compounds. Molecular covalent compounds also take part in oxidation-reduction reactions.

COMBUSTION: REDOX REACTION

CH₄ + 2O₂ → CO₂ + 2H₂O

In this reaction, carbon is oxidized. Electron transfer is trickier to identify in reactions with covalent compounds. But we already know that combustion is a form of oxidation. For molecules, oxidation often involves a loss of hydrogen and a gain of oxygen, and reduction often involves a gain of hydrogen and the loss of oxygen.

LESSON SUMMARY

What happens to electrons during oxidation?

Whenever an atom or ion loses electrons, another atom or ion gains electrons. The loss of electrons is called oxidation, and the gain of electrons is called reduction. Oxidation and reduction always occur together. This process is known as oxidation-reduction, or redox. When metals react with oxygen to form metal oxides, the metal atoms are oxidized and the oxygen is reduced. To extract elemental metals, the reverse reaction must occur and metal ores must be reduced.